An astronaut propulsion unit (or astronaut maneuvering unit) is used to move an astronaut relative to the spaceship during a spacewalk. The first astronaut propulsion unit was the Hand-Held Maneuvering Unit (HHMU) used on Gemini 4. Models Hand-Held Maneuvering Unit The Hand-Held Maneuvering Unit was the EVA "zip" gun used by Ed White on the Gemini 4 mission in 1965. The hand-held gun held several pounds of nitrogen, and allowed limited movement around the Gemini spacecraft. It was also used by astronaut Michael Collins on the Gemini 10 mission in 1966. USAF Astronaut Maneuvering Unit The United States Air Force (USAF) Astronaut Maneuvering Unit (AMU) was designed by the U.S. Air Force, which was planning to use the Gemini spacecraft as part of the Manned Orbiting Laboratory (MOL). The AMU was a backpack using hydrogen peroxide as the fuel. The total delta-v capability of the AMU was about 250 feet per second (76.2 meters per second), roughly three times that of the MMU. The astronaut would strap on the AMU like a backpack, and maneuver around using two hand controllers like that of the later MMU. Because of the fuel, which came out as a hot gas, the astronaut's suit had to be modified with the addition of woven metal "pants" made of Chromel-R metal cloth. The AMU was carried aboard the Gemini 9 mission, but was not tested because the astronaut, Eugene Cernan, had difficulty maneuvering from the Gemini cabin to the AMU storage place, at the back of the spacecraft, and overheated, causing his helmet faceplate to fog up. The AMU was also meant to be launched and flown on-board Gemini 12, and to fly untethered from the Gemini spacecraft, but was scrubbed two months before the mission. NASA chief astronaut Deke Slayton later speculated in his autobiography that the AMU may have been developed for the MOL program because the Air Force "thought they might have the chance to inspect somebody else's satellites." Automatically Stabilized Maneuvering Unit (ASMU) In 1973, the Automatically Stabilized Maneuvering Unit (ASMU) was test-flown aboard Skylab during the Skylab 3 and 4 missions. Tested inside of the orbiting laboratory, it used nitrogen gas allowing both unsuited and suited testing of the unit. The Skylab AMU was the closest to the Shuttle MMU, but was not used outside the spacecraft because the EVAs were conducted with the astronauts attached to life support umbilicals, and